Thin and Small N-doped Carbon Boxes Obtained from Microporous

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Thin and Small N-doped Carbon Boxes Obtained from Microporous Organic Networks and Their Excellent Energy Storage Performance at High Current Densities in Coin Cell Supercapacitors Junpyo Lee, Jaewon Choi, Daye Kang, Yoon Myung, Sang Moon Lee, Hae Jin Kim, Yoon-Joo Ko, Sung-Kon Kim, and Seung Uk Son ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03836 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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ACS Sustainable Chemistry & Engineering

Thin and Small N-doped Carbon Boxes Obtained from Microporous Organic Networks and Their Excellent Energy Storage Performance at High Current Densities in Coin Cell Supercapacitors Junpyo Lee,†,‡ Jaewon Choi,†,‡ Daye Kang,† Yoon Myung,∫ Sang Moon Lee,§ Hae Jin Kim,§ Yoon-Joo Ko,∞ Sung-Kon Kim,,* and Seung Uk Son†,* †

Department of Chemistry, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Korea E-mail address: Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Korea § Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Korea ∞ Laboratory of Nuclear Magnetic Resonance, National Center for Inter-University Research Facilities (NCIRF), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  School of Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 54896, Korea Corresponding author* E-mail address: [email protected], [email protected] ∫

Supporting Information

ABSTRACT: This work reports the thinnest and smallest hollow NHeat doped carbon boxes. Hollow and N-rich microporous organic networks Thin Shell Hollow NMON Ar (H-NMONs) were prepared by the azide-alkyne Huisgen cycloaddition of tetra(4-ethynylphenyl)methane and 1,4-diazidobenzene on the sur~12 nm face of Cu2O nanocubes and the successive acid etching of inner Cu 2O. 130 nm The Cu2O nanocubes played roles of templates and networking cataThin Shell lysts. The networking reaction generated N-rich triazole rings in the N-doped Carbon MON. Heat treatment of H-NMONs under argon resulted in the forN Small Box N mation of hollow N-doped carbon boxes (H-NCBs). The diameter and N N-rich shell thickness of H-NCBs were 130 and 12 nm, respectively. The HMicroporous NCBs showed superior electrochemical performance in H 2SO4 as enerOrganic Network N (NMON) gy storage materials for supercapacitors, compared with that in KOH. N N N N N Among the H-NCBs, H-NCB-900 which was obtained by the heat treatment of H-NMON at 900 oC showed the best performance with Coin Cell Type capacitances of 286 and 251 F/g at current densities of 1 and 10 A/g in Supercapacitor two electrode coin cell type supercapacitors and maintained the capacitances of 228 and 206 F/g at higher current densities of 50 and 100 A/g. Moreover, the H-NCB-900 showed excellent cycling stabilities with ~95% retention of the first capacitance after 10000 cycles. The excellent electrochemical performance of H-NCBs can be attributed to their efficient N-doping, hollow structure, and thin thickness of shells. N N N

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KEYWORDS: Microporous organic network, Polymer, Hollow material, N-doped carbon, Supercapacitor  INTRODUCTION Due to their high power densities and fast charge/discharge processes, various supercapacitors have been engineered using carbon-based energy storage materials.1 In continuing efforts to improve storage performance, the morphology engineering of carbon materials has attracted attention of scientists.2 For examples, hollow morphologies of carbon materials can be beneficial because thin shells induce facile mass diffusion of electrolytes into materials, maximizing the utilization of storage materials.3 Moreover, the doping of heteroatoms to carbon

materials is also an attractive strategy to improve the storage performance because heteroatoms such as nitrogen not only can tune the electronic properties of carbon materials and but also can act as redox active sites to result in pseudocapacitive performance.4-5 Thus, hollow N-doped carbon materials are attractive materials and thus, have been engineered.6-18 However, further synthetic exploration for hollow N-doped carbon materials is required not only for methodological diversity19-20 but also for more delicate control of material parameters such as thin shell fabrication.

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Recently, there have been efforts to find new carbon precursors and new synthetic chemistry for the development of energy storage materials.21-31 For a recent example, Cooper and coworkers reported the application of conjugated microporous polymers (CMPs) as precursors of carbon materials for supercapacitors.32 These CMPs were prepared by the Sonogashira coupling of aryl alkynes and aryl halides.33-36 Because chemical components of CMPs can be easily controlled by changing the building blocks, nitrogen could be introduced into CMPs via a pre-designed building block approach. Moreover, posttreatment of CMPs with ammonia gas can introduce N contents into materials. The pyrolysis of N-containing CMPs resulted in N-doped carbon materials that showed promising performance as energy storage materials for supercapacitors. 32 Our research group has shown that microporous organic networks (MONs) can be engineered by template methods.37-38 The functions of MONs can be improved through engineering of shapes.39 For example, hollow MON catalysts showed the enhanced activities due to the facile diffusion of substrates into MON networks. Same morphological benefits can be applied to energy storage materials in supercapacitors. 40 The MONs can be prepared by various coupling reaction of building blocks.41-46 Nitrogen can be incorporated into MONs based on the azide-alkyne Huisgen cycloaddition reaction (click reaction) of organic building blocks.49-51 Because the connection of aryl azides and aryl alkynes results in triazole rings with three Ns in each ring, the resultant MON materials are N-rich. Moreover, hollow N-rich MON materials can be prepared using Cu2O nanomaterials as networking catalysts and templates.52 The pyrolysis of hollow N-rich MON materials may result in N-doped carbon materials. In this work, we report the synthesis of the thinnest and smallest hollow N-doped carbon nanoboxes (H-NCBs) by pyrolysis of hollow MON boxes rich in triazole rings (HNMONs) and their enhanced electrochemical performances as energy storage materials for supercapacitors.  EXPERIMENTAL SECTION General Information. Scanning electron microscopy (SEM) was conducted using a JSM6700F instrument. Powdery samples were loaded on a carbon tape for SEM analysis. Transmission electron microscopy (TEM) was conducted using a JEOL 2100F. For TEM analysis, the powdery samples were dispersed in methanol and drop-casted on a carbon film on a copper grid. Powder X-ray diffraction (PXRD) studies were conducted using a Rigaku MAX-2200 (Cu-K radiation) instrument. N2 adsorption-desorption isotherm curves were obtained at 77K using a BELSORP II-mini equipment. The isotherm curves were analyzed based on the Brunauer-EmmettTeller (BET) theory. The pore size distribution was analyzed based on the density functional theory (DFT). Thermogravimetric analysis (TGA) was conducted using a Seiko Exstar 7300. Solid state 13C nuclear magnetic resonance (NMR) spectroscopy was conducted with a mode of crosspolarization/total side band suppression (CP-TOSS) using a 500 MHz Bruker ADVANCE II NMR spectrometer. The spin rate was 5 kHz. Infrared absorption (IR) spectroscopy was conducted using a Bruker VERTEX 70 FT-IR instrument. Raman spectroscopy was conducted using a Renishaw inVia Raman Instrument. The peaks were analyzed by Gaussian function using a PeakFIT 4.12 program. X-ray photoelectron

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(XPS) spectroscopy was conducted using a Thermo VG and Al-K radiation. Elemental analysis was conducted using a CE EA110 analyzer. Electrochemical studies were conducted using a WonAtech ZIVE SP1 Electrochemical Workstation. Synthetic Procedures for H-NMON. To be used as templates and networking catalysts, Cu2O nanocubes were prepared by the synthetic procedures reported in the literature.53 In our work, the following procedures were applied. Distilled water (800 mL) was added to a 1 L round bottomed flask. Aqueous solution of trisodium citrate (0.90 M, 4.0 mL, 3.6 mmol trisodium citrate) was added to the flask. After stirring for 20 min, aqueous solution of CuSO4 (1.20 M, 4.0 mL, 4.8 mmol CuSO4) was added to the solution in the flask. After stirring for 5 min, aqueous solution of NaOH (4.80 M, 4.0 mL, 19 mmol NaOH) was added to the solution in the flask. After stirring for 5 min, aqueous solution of ascorbic acid (1.20 M, 4.0 mL, 4.8 mmol, ascorbic acid) was added to the solution in the flask. After stirring for 30 min, Cu2O powder was separated by centrifugation, washed three times with methanol (40 mL each), and dried at room temperature under vacuum. For the synthesis of Cu2O@NMON, Cu2O nanocubes (0.20 g) were dispersed in a mixture of dimethylsulfoxide (DMSO, 31 mL) and H2O (4 mL) in a 50 mL Schlenk flask. The mixture was sonicated for 1 h. Tetra(4-ethynylphenyl)methane54 (20 mg, 48 mol) and 1,4-diazidobenzene55 (15.5 mg, 96 mol) were dissolved in DMSO (5 mL). This solution was added to the mixture containing Cu2O nanocubes and heated at 80oC for 20 h. After cooling the mixture to room temperature, the powder was separated by centrifugation, washed three times with acetone (40 mL each), three times with a 1:1 mixture of methylene chloride and hexane (40 mL each), and dried at room temperature under vacuum. The obtained Cu 2O@NMON was added to HCl aqueous solution (2 M, 40 mL) in an 80 mL Falcon tube. After stirring for 30 min, the H-NMON was separated by centrifugation, washed four times with a 1:1 mixture of methanol and water (40 mL each), and was transferred to a 20 mL vial. The H-NMON was further washed two times with a 1:1 mixture of methanol and methylene chloride (10 mL each) and dried at room temperature under vacuum. Synthetic Procedures for H-NCBs. H-NMON (45 mg) was loaded on a crucible and added to a furnace filled with argon. After argon flowed for 30 min, the temperature in the furnace was gradually increased to 900oC (temperature increase rate: 5oC/min). Then, the sample was treated at 900oC for an additional 3 h and cooled to room temperature. The resultant product was denoted at H-NCB-900. For the synthesis of H-NCB600, H-NCB-700, and H-NCB-800, the treating temperatures were adjusted to 600, 700, and 800oC, respectively. Other procedures were the same as those applied for H-NCB-900. Procedures for Electrochemical Studies. For the fabrication of working electrodes, H-NCB (20 mg), Super P carbon black (2.5 mg), and polyvinylidene fluoride (PVDF 2.5 mg, 19 mg of 13% PVDF in NMP) were dispersed in N-methylpyrrolidinone (NMP, 25 mg) by grinding using a mortar and pestle. Using a spatula, the slurry was coated on the surface of a Ti foil (0.77 cm  0.64 cm, 0.127 mm thick, annealed, Alfa Aesar Co.), dried at 80oC in an oven for 3 h, and then dried overnight at 100oC in a vacuum oven. The loading amount of H-NCB was calculated as 0.97 mg/cm2. The thicknesses of


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electrode materials were measured as 58.1, 57.5, 57.4, and 59.7 µm for H-NCB-600, H-NCB-700, H-NCB-800, and HNCB-900, respectively, using a thickness gauge (Mitutoyo Co.). Two working electrodes were prepared for the fabrication of coin-cell type supercapacitors. One working electrode was loaded on one cap of CR2032 cell. Electrolyte solution (1 M H2SO4 or 6 M KOH aqueous solution, 80 mg) was loaded on the working electrode. A separator (No. 20 filter paper, HYUNDAI MICRO Co., HD20 MN020 5~8 m) was loaded on the working electrode. The second working electrode was loaded on the separator. The active material on the second working electrode was located in the direction to the separator. Electrolyte solution (1 M H2SO4 or 6 M KOH aqueous solution, 80 mg) was loaded on the working electrode. A space disc, a spring, and the other cap of CR2032 cell were loaded on the second working electrode. Using a crimper, coin-cells were assembled. Electrochemical tests were conducted after the cells standing for 24 h. Cell voltages were scanned in a range of 0 ~ 1 V. Cell capacitance (Ccell) was calculated by the following equation (1) : Ccell = I/[(ΔV/Δt)m] Eq. (1), where I is the applied current (A), ΔV/Δt is the slope of discharge curves after a IR drop at the beginning of the discharge curve, and m is the total mass of electrode materials (g). The specific capacitance of the single electrode was calculated by the equation (2), Cs = 4Ccell Eq. (2).56  RESULTS and DISCUSSION A synthetic scheme for H-NCBs is shown in Figure 1. The copper-catalyzed click reaction is a facile coupling reaction of aryl azide and aryl alkyne.57-59 MON materials have been prepared by this reaction.49-51 Moreover, using silica templates, hollow MON spheres were engineered.60 Various copper species are known to catalyze this reaction. Among Cu(+1) species, Cu2O powder has been used to catalyze this reaction.61-64 In addition, Cu2O nanomaterials have been engineered to show enhanced catalytic activities.53,61-64 Thus, the Cu2O nanomaterials can be used not only as catalysts for the synthesis of MONs based on the click reaction but also as templates.52 For using as nanotemplates, Cu2O nanocubes were prepared by the synthetic procedures reported in the literature.53 MON layers were formed on the surface of Cu 2O nanocubes by the click reaction of 1 eq. tetra(4-ethynylphenyl)methane54 with 2 eq. 1,4-diazidobenzene. Because the surface of Cu2O nanocubes is catalytically active for the click reaction of building blocks, the MON layers were formed exclusively on the surface of Cu2O nanocubes. The inner Cu2O could be etched by the treatment of hydrochloric acid to form hollow MON boxes bearing triazole rings (H-NMONs). The heat treatment of H-NMONs at 600, 700, 800, and 900 oC resulted in the HNCB-600, H-NCB-700, H-NCB-800, and H-NCB-900, respectively.

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The Cu2O nanotemplates and the synthesized intermediate/final materials were investigated by scanning (SEM) and transmission electron microscopies (TEM). As shown in Figure 2a, the Cu2O nanomaterials had a cube shape with a diameter of 105 nm. The SEM image of Cu2O@NMON showed a turbid contrast due to the formation of organic layers on the Cu2O materials, compared with that of Cu2O nanocubes. (Figure 2b) The magnified SEM images of Cu2O@NMON showed uniform coating of MON layers on the Cu 2O nanocubes. (Figure S1 in the SI) The thicknesses of the MON layers were measured in the range of 12~17 nm. The SEM and TEM images of H-NMONs showed the hollow nature of materials with a shell thickness of ~15 nm and a diameter of ~135 nm, matching well with the original thickness of NMON in Cu2O@NMON. (Figure S2 in the SI) The carbon materials of H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900 showed the retention of the original box shape and shell thicknesses of H-NMON materials. (Figures 2c-h and S3 in the SI) The shell thicknesses and diameters of H-NCB-600, H-NCB700, H-NCB-800, and H-NCB-900 were nearly same and were observed as ~12 nm and ~130 nm, respectively.



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The physical and chemical properties of materials were investigated by various analysis methods. According to analysis of N2 adsorption-desorption isotherm curves based on the Brunauer-Emmett-Teller (BET) theory, the Cu2O nanocubes showed a surface area of 16 m2/g. The Cu2O@NMON showed an increase of a surface area to 132 m2/g through the formation of MON layers with high surface areas, compared to the nonporous Cu2O nanocubes. (Figure 3a) The H-NMON materials showed a surface area of 886 m2/g, supporting the high porosity of materials. The analysis of pore size distribution of HNMON based on the density functional theory (DFT) showed microporosity (main pore sizes < 2 nm) with a micropore volume (Vmic) of 0.25 cm3/g. The surface areas of H-NCB-600, HNCB-700, H-NCB-800, and H-NCB-900 were observed as 643 (Vmic: 0.23 cm3/g), 496 (Vmic: 0.16 cm3/g), 455 (Vmic: 0.15 cm3/g), and 443 m2/g (Vmic: 0.14 cm3/g), respectively, indicating the decomposition of MON materials and the generation of new micropores in carbon materials. (Figure 3b) The powder X-ray diffraction (PXRD) studies on the Cu2O and Cu2O@MON showed (110), (111), (200), (220), (310), and (222) diffraction peaks at 29.7, 36.6, 42.5, 61.5, 73.7, and 77.5o (2θ value), respectively, indicating that crystalline materials are cubic phase Cu2O (JCPDS# 78-2073). (Figure 3c) The H-NMON showed amorphous characteristic, which is a conventional property of MON materials prepared by the click coupling reaction of organic building blocks.49-51 All H-NCBs also showed conventional amorphous characteristic. (Figure 3d)

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Figure 3. (a-b) N2 adsorption-desorption isotherm curves at 77K, pore size distribution diagrams based on the DFT method, (c-d) PXRD patterns of Cu2O, Cu2O@NMON, H-NMON, and H-NCBs. (e) Solid state 13C NMR spectrum of H-NMON.

Solid state 13C nuclear magnetic resonance (NMR) spectroscopy of H-NMON showed the 13C peak of quaternary aromatic carbon of triazole rings and benzyl carbon originating from the tetrahedral building blocks were observed at 147 and 64 ppm, respectively. (Figure 3e) The 13C peaks from other aromatic rings were observed at 121, 126, and 135 ppm. The 13C spectrum of H-NMON matches well with the expected chemical structure and those of the MONs prepared by the click reaction in the literature.49-51 Infrared (IR) spectroscopy on the HNMON showed the –N=N- vibration peak of triazole rings at 1607 cm-1, matching with that of MONs prepared by the click reaction in the literature.49-51,65 (Figure S4 in the SI) Elemental analysis of H-NMON by combustion showed N contents of 18.20 w%. The N contents in H-NCB-600, H-NCB-700, HNCB-800, and H-NCB-900 were observed as 5.51, 3.77, 3.23, and 2.89 wt%, respectively. Thermogravimetric analysis (TGA) showed that thermal stability of H-NCB increases with an increase of carbonization temperature. (Figure S5 in the SI) The chemical surroundings of nitrogen in N-doped carbon materials are critical in their storage performance for superca-


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pacitors. Thus, H-NCBs were further investigated by X-ray photoelectron (XPS) and Raman spectroscopy. In XPS spectra, the N 1s orbital peaks of pyridinic, pyrrolic, graphitic, and oxide nitrogen in H-NCBs appeared at 398.3, 399.7, 400.8, and 402.1 eV, respectively.5 (Figure 4a) As carbonization temperature increased form 600oC to 700, 800, 900oC, the graphitic N portion in N contents of H-NCBs increased from 39% to 41, 47, and 51%, respectively. In contrast, the pyridinic N portion in N contents of H-NCBs gradually decreased from 36% to 30, 25, and 23%, respectively. Also, the pyrrolic N portions in N contents of H-NCBs gradually decreased from 21, 20, 18, and 14%, respectively. The location of N 1s orbital XPS peaks of H-NMON was significantly different from those of HNCBs and analyzed by a 1:1:1 combination of three peaks observed at 399.4, 400.2, and 401.5 eV, matching well with the fact that the triazole ring contains three chemically different Ns. (Figure 4a) The Raman spectra showed that when carbonization temperature increased from 600oC to 700oC, the ID/IG ratio of D (disordered carbon) and G (graphitic carbon) band intensities at 1349 and 1585 cm-1 increased from 0.67 to 0.81, indicating that N-doping induced disorder in carbon materials could be significantly formed at around 700oC. The ID/IG ratios were maintained at 0.83 and 0.84 in H-NCB-800 and H-NCB-900, respectively. (Figure 4b) The ID/IG ratios of most N-doped hollow carbon materials in the literature were reported in the range of 0.92~1.5.6-18 The lower ID/IG ratios of H-NCB-700, H-NCB-800, and H-NCB-900, compared to those of N-doped hollow carbon materials in the literature6-18 imply more contents of sp2 graphitic carbon and higher conductivities. Thus, the H-NCBs were expected to show good conductivities and promising rate performance at high current densities as energy materials for supercapacitors.

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Considering the N-doping and the thin shell thicknesses of H-NCBs, we studied their energy storage performance for supercapacitors. Figures 5-6 and S6-8 in the SI summarize the results.

Figure 5. Electrochemical performance of symmetrical coin cell type supercapacitors. Cyclic voltammograms (scan rate: 100 mV/s) of H-NCBs in (a) 6 M KOH and (b) 1 M H2SO4. Charge/discharge profiles (current density: 1 A/g) of H-NCBs in (c) 6 M KOH and (d) 1 M H2SO4. (e) Rate performance of H-NCBs in 6 M KOH (blank circle) and 1 M H2SO4 (solid circle). Nyquist plots of HNCBs in (f) 6 M KOH and (g) 1 M H2SO4.

We fabricated coin cell type (CR2032) two electrode supercapacitors using H-NCBs (loading amount: ~1 mg/cm2). Overall, as the carbonization temperature increased from 600oC to 700, 800, and 900oC, the capacitances of H-NCBs gradually increased from 40 F/g to 189, 333, and 365 F/g (current densi-



ty: 0.1 A/g), respectively. (Figures 5a-b, and 5e) Among the H-NCBs, the H-NCB-900 showed the best performance in capacitance. When we increased the loading amount of HNCB-900 from 0.97 mg/cm2 to 2, 3, and 4 mg/cm2, capacitances decreased from 286 F/g to 202, 165, and 125 F/g (current density: 1 A/g). (Figure S8 in the SI) We found that H-NCBs showed very different performance depending on the electrolytes: basic 6 M KOH and acidic 1 M H2SO4. (Figure 5) The cyclic voltammograms (CVs) of HNCB-900 were relatively more rectangular in the KOH electrolyte, compared with those with additional redox peaks in the H2SO4 electrolyte.4-18 (Figures 5a-b and S5-6 in the SI) The charge/discharge profiles of H-NCBs were more symmetrical in the KOH electrolyte than those in the H2SO4 electrolyte, indicating that while the storage mechanisms of H-NCBs in the KOH are closer to conventional electrostatic double layer capacitive storage, those in the H2SO4 indicate additional pseudocapacitive behaviors. (Figures 5a-d) We suggest that the enhanced performance of H-NCBs in the H2SO4, compared with that in the KOH is attributable to more efficient chemical attraction (acid-base reaction) of the acid electrolyte with basic N moieties in H-NCBs. In addition, the thin shell thickness of H-NCBs can enhance the difference. The H-NCBs showed clearly better rate performance in the H2SO4 electrolyte than those in the KOH electrolyte. (Figure 5e) Interestingly, the capacitances of H-NCB-900 at a current density of 0.3 A/g are the same as 294 F/g in the cases of the H2SO4 and the KOH electrolytes, respectively. The capacitances of H-NCB-900 in H2SO4 maintained the 286 (97%), 251 (85%), 244 (83%), 228 (78%), and 206 F/g (70%) at the current densities of 1, 10, 20, 50, and 100 A/g, respectively, compared with that at current density of 0.3 A/g. In contrast, the capacitances of H-NCB-900 in KOH significantly decreased to the 270 (92%), 225 (77%), 206 (70%), 159 (54%), and 116 F/g (39%) at the current densities of 1, 10, 20, 50, and 100 A/g, respectively, compared to that at current density of 0.3 A/g. These results imply that the H-NCBs have higher conductivities in H2SO4, compared with those in KOH. The Nyquist plots were obtained through the electrochemical impedance spectroscopy (EIS), showing that as the carbonization temperature increased from 600oC to 700, 800, and 900oC, charge transfer resistances (Rct) of H-NCBs in KOH electrolyte gradually decreased from > 100 Ω to 62, 8.7, and 6.0 Ω, respectively. (Figure 5f) In comparison, as the carbonization temperature increased from 600oC to 700, 800, and 900oC, the charge transfer resistances of H-NCBs in H2SO4 electrolyte gradually decreased from > 100 Ω to 4.5, 1.6, and 0.98 Ω, respectively. (Figure 5g) The higher conductivities of H-NCBs in H2SO4 electrolyte compared with those in KOH electrolyte are attributable to the efficient interaction of acidic electrolyte with basic N-doped carbon materials, possible to form ammonium salt moieties in the H-NCBs. Next, cycling tests were conducted to study the electrochemical stability of H-NCB-900 as a representative material. As shown in Figure 6, the coin cell type supercapacitors of HNCB-900 showed good cycling performance at various current densities. At the current densities of 20, 50, and 80 A/g, the cells of H-NCB-900 in H2SO4 electrolyte showed capacitances of 244, 233, and 215 F/g at the first cycle. After 10000 cycles, the cells maintained 231 (94.7% of the first cycle), 221 (94.8% of the first cycle), and 205 F/g (95.3% of the first cycle), at the

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current densities of 20, 50, and 80 A/g, respectively. The supercapacitor cells recovered after cycling tests maintained the high conductivity with a charge transfer resistance of 1.6 Ω. (Figure S9 in the SI) 94.7%

20 Ag-1

50 Ag-1

80 Ag-1

94.8% 95.3%

Figure 6. Cycling tests of coin-cell type supercapacitors of HNCB-900 at the current densities of 20, 50, and 80 A/g (electrolyte: 1 M H2SO4).

We surveyed the electrochemical performances of the related N-doped carbon materials for supercapacitors reported in top journals and compared those with our results.6-18,32,66-67 (Figure 7 and Table S1 in the SI) In the cases of the synthesis of nonhollow N-doped carbon materials using microporous organic polymer materials as precursors, very recently, Cooper and co-workers reported the synthesis of N-doped carbon materials by the thermolysis of CMPs which were prepared by the Sonogashira coupling of organic building blocks.32 The materials showed a capacitance of 149 F/g at a current density of 10 A/g in a three electrode system using 1 M H2SO4 as an electrolyte. In another example, Zhi and co-workers reported the synthesis of N-doped carbon powders by the thermolysis of Nrich porous organic polymers which were prepared by the cross-linking of terephthalonitrile monomers.66 The resultant N-doped carbon materials showed specific capacitance up to 173 F/g (current density: 10 A/g) in two electrode cells using 1 M H2SO4 as electrolyte. In comparison, H-NCB-900 in the present work showed a specific capacitance of 251 F/g (current density: 10 A/g) in two electrode systems with 1 M H2SO4 electrolyte. In the very recent synthetic case, Lou and co-workers reported the synthesis of hollow N-doped carbon materials by the thermolysis of the porous zeolitic imidazolate framework (ZIF-8) and showed a specific capacitance of 252 F/g at the current density of 10 A/g in a two electrode system using 2 M H2SO4 as electrolyte.67 While this performance is nearly same with ours (251 F/g at 10 A/g), the materials in the literature showed capacitances of 235 and 193 F/g at the current densities of 20 and 50 A/g, respectively. In comparison, H-NCB900 in present work showed capacitances of 244, 228, and 206 F/g at the current densities of 20, 50, and 100 A/g. As far as we are aware, our work may be the first example of the engineering of hollow N-doped carbon materials using the microporous polymer materials as precursors. The enhanced electrochemical performance of H-NCBs is attributable to several factors, i.e., the efficient N-doping as a result of building block approach, the well distribution of N moieties, the good conductivity (Rct: upto 0.98 Ω), and the hollow structure.


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Especially, the thickness (12 nm) and diameter (130 nm) of HNCB-900 are thinnest and smallest, respectively, among the recent hollow N-doped carbon materials6-18,32,66-67 (excepting 2D materials14) (Figure 7 and Table S1 in the SI), which are beneficial in the efficient contact of materials with an acidic electrolyte and in the ultimate utilization of electrochemically active sites.3 (a)

(b)

‡ These authors contributed equally to this work. The authors declare no competing financial interests.

 ACKNOWLEDGMENT This work was supported by Basic Science Research Program (2016R1E1A1A01941074) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning and the grants CAP-15-02KBSI (R&D Convergence Program) of National Research Council of Science & Technology (NST) of Korea.  REFERENCES 1

This work

2 This work

This work

Figure 7. Comparison of (a) electrochemical performance and (b) size parameters of H-NCB-900 and the recent N-doped hollow carbon materials6-8,32,66-67 used for supercapacitors (Refer to Table S1 in the SI for detail values.).

 CONCLUSION This work shows that the Cu2O template synthesis of hollow N-rich MONs can be applied for the engineering of energy storage materials for supercapacitors. Azide-alkyne Huisgen cycloaddition of building blocks resulted in N-rich triazole moieties on the surface of templates. The homogeneous distribution of triazole rings over MONs can be applied for the synthesis of N-doped carbon materials. Moreover, the template synthesis resulted in the very thin shell N-doped carbon materials with hollow box shapes. The resultant H-NBC-900 showed excellent electrochemical performance with capacitances of 286, 251, 228, and 206 F/g at the current densities of 1, 10, 50, and 100 A/g, respectively, in two electrode coin cell supercapacitors. The H-NBC-900 showed excellent cycling stabilities with ~95% retention of the capacitance of the first cycle after 10000 cycles. Interestingly, the H-NBCs showed clearly superior electrochemical performance in an acidic electrolyte, compared with basic one. We suggest that these features originate from the efficient N-doping and the thin shell thickness of H-NCBs. We believe that the chemical/physical parameters of H-NCBs can be further optimized by the screening the organic building blocks.  ASSOCIATED CONTENT Supporting Information. SEM images of Cu2O@NMON and H-NMON, IR spectrum of H-NMON, cyclic voltammograms of H-NCBs, and comparison of electrochemical performance of H-NCBs with the recent N-doped hollow carbon materials. This material is available free of charge via the Internet at http://pubs.acs.org.  AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Notes

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Table of Contents Hollow and N-rich microporous organic networks with thin shells showed capacitances of 286 ~ 206 F/g (1 ~ 100 A/g) in coin cell type supercapacitors. N N N

N N N

N

N

N

N

N

N

N N

Thin Shell Hollow NMON N N

N

N

N-rich Microporous Organic Network N (NMON) N

N N N

N N

N

N

N N N

N

N

Heat

N

Ar

N

Coin Cell Type Supercapacitor N

N N

N

N

N

Thin Shell N-doped Carbon Small Box

N N

~12 nm N N

N

N N N

130 nm


10

Thin and Small N-doped Carbon Boxes Obtained from Microporous

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